1. Field of the Invention
The present invention relates to a disk control system and, more particularly, to a disk control system for controlling a plurality of magnetic disk drives.
2. Description of the Related Art
Generally, in a disk system including a plurality of magnetic disk drives, preventative maintenance for each disk drive is performed in order to ensure the reliability of an operation of the disk drive. That is, inspection of each disk drive or its operation check is performed periodically. Upon occurrence of a failure, recovery processing for the failure or parts replacement is executed. Execution of this preventative maintenance can prevent occurrence of an event such as, for example, a disk system in operation going down due to a failure.
In this preventative maintenance, a check work for the operation of a magnetic disk drive is performed by executing the following three failure countermeasure functions; i.e., a failure detection function, a failure recovery function, and a failure prevention function.
The failure detection function performs error detection and correction for data recorded on a magnetic recording medium of a magnetic disk drive by means of parity check, Hamming code check, or cyclic code check.
The failure recovery function performs appropriate processing for a detected malfunction to recover a normal state. For example, reformatting or the like of a magnetic disk drive corresponds to this function.
The failure prevention function detects a failure beforehand during an operation of a magnetic disk system. A patrol function (patrol seek/verify), for example is known as this type of failure prevention function. This patrol function periodically reads out data in sequence from magnetic disk drive independently of access by a user program and finds a failure before it happens, in accordance with the presence or absence of an error in the readout data. Information concerning, e.g. the location where a failure found by the patrol function took place, is sampled as error log information, and this error log information is used to carry out the work of preventative maintenance.
In executing the preventative maintenance work using the above failure detection, recovery, and prevention functions, it is a common practice to set a magnetic disk drive as an object of the maintenance in an inoperative state and to inhibit all I/O requests from user programs to the magnetic disk drive of interest.
This is so because when the recovery function, such as reformatting, for example, is to be performed in the preventative maintenance operation, it is necessary to save and store data in the magnetic disk drive of interest in another memory and to restore the saved data to the magnetic disk drive after reformatting. In this case, since neither data write nor read can be performed for that disk drive during reformatting, all I/O requests from user programs are inhibited.
Also, even while a disk drive is being used, if contention occurs between access by the patrol function and an I/O request from a user program, the I/O request from the user program is waited. This results in degradation in the I/O performance with respect to that disk drive.
As described above, in conventional systems, I/O requests from user programs must be sacrificed for the operation check while performing preventative maintenance for magnetic disk drives. As a result, the operating performance of the magnetic disk system is reduced.
Especially in a large-capacity magnetic disk system including a large number of magnetic disk drives, it is necessary to execute the operation check for preventative maintenance for all of the magnetic disk drives. The result is that a very long time is required for the preventative maintenance of the magnetic disk system. For this reason, the time during which the magnetic disk system is set in an inoperative state is prolonged, causing an increase in the number of I/O requests from user program whose acceptance is inhibited.
A failure caused by head crash also is known as a problem inherent in a magnetic disk drive of a magnetic disk system.
Head crash is a phenomenon in which a magnetic head and a magnetic recording medium mechanically collide against each other due to dust, and consequently the head or the recording medium is damaged.
A failure caused by head crash takes place because a magnetic disk drive adopts a CSS (contact-start-stop) system; that is, a magnetic head is slightly floated from a recording medium rotated at high speed. The floating position of the magnetic head is unbalanced by dust, and this causes mechanical contact between the magnetic head and the recording medium.
Especially when a magnetic head is kept located on a specific recording area (track) and is not moved to another area (this state is called a disk fixed position floating state), dust easily accumulates on the recording medium, allowing easy occurrence of head crash. For this reason, if only a particular address of a disk drive is accessed frequently, this disk fixed position floating state easily increases the possibility of head crash.
In order to prevent failure caused by such head crash, a means for, e.g., increasing dust removal efficiency by performing ventilation using an air filter is used. However, this countermeasure is in practice unsatisfactory when the disk fixed position floating described above is taken into consideration. Therefore, it is difficult to eliminate the danger of head crash.